System and method for controlling a converter

ABSTRACT

A system and method for controlling a converter. One embodiment provides the cyclic actuation of a first switching element, used for applying an input voltage to an inductive storage element. A second switching element is used as a first rectifier element in a rectifier arrangement, in a step-up converter. An actuating circuit is provided for the first and second switching elements.

BACKGROUND

Step-up converters (boost converters) have an inductive storage element,a switching element for the clocked application of an input voltage tothe inductive storage element, and a rectifier arrangement which isconnected to the inductive storage element. The rectifier arrangementhas a diode as a proficient rectifier element and a capacitive storageelement which provides an output voltage.

During operation of such a step-up converter, the switching element isturned on cyclically for a turned-on period. During this turned-onperiod, energy is stored in the inductive storage element and theinductive storage element is magnetized thereby. When the switchingelement has been turned off, this stored energy is output to therectifier arrangement.

In one known method the magnetization state of the inductive storageelement is monitored and the switching element is respectively turned onagain as soon as the inductive storage element is completelydemagnetized. During the turned-off period of the switching element, theswitching element has a voltage across it which correspondsapproximately to the output voltage of the step-up converter. If theswitching element has a parasitic capacitance, this capacitance ischarged to the output voltage during the turned-off period and isdischarged when the switching element is turned on again. This processresults in switching losses, which are greater the greater the parasiticcapacitance of the switching element.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of embodiments and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments andtogether with the description serve to explain principles ofembodiments. Other embodiments and many of the intended advantages ofembodiments will be readily appreciated as they become better understoodby reference to the following detailed description. The elements of thedrawings are not necessarily to scale relative to each other. Likereference numerals designate corresponding similar parts.

FIG. 1 illustrates one embodiment of a basic circuit for a step-upconverter using an electrical equivalent circuit diagram.

FIG. 2 illustrates one embodiment of time profiles for signals whichoccur in the step-up converter during an actuation cycle of an actuationmethod.

FIG. 3 illustrates one embodiment of an actuating circuit in a step-upconverter.

FIG. 4 illustrates one embodiment of a signal generation circuit whichis present in the actuating circuit.

FIG. 5 illustrates one embodiment of a signal generation circuit whichis present in the actuating circuit.

FIG. 6 illustrates a peak reverse recovery current value dependency onan input voltage for an example of an actuation method.

FIG. 7 illustrates one embodiment of time profiles for signals occurringin the step-up converter during an actuation cycle.

FIG. 8 illustrates a peak reverse recovery current value dependency on acontrol signal for an example of an actuation method.

FIG. 9 illustrates one embodiment of a switch-mode converter arrangementwhich has a plurality of switch-mode converters connected in parallel.

FIG. 10 illustrates the profile of an effective capacitance which ispresent when compensating components are used as switching elements.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., is used withreference to the orientation of the Figure(s) being described. Becausecomponents of embodiments can be positioned in a number of differentorientations, the directional terminology is used for purposes ofillustration and is in no way limiting. It is to be understood thatother embodiments may be utilized and structural or logical changes maybe made without departing from the scope of the present invention. Thefollowing detailed description, therefore, is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

It is to be understood that the features of the various exemplaryembodiments described herein may be combined with each other, unlessspecifically noted otherwise.

One embodiment relates to a method for the cyclic actuation of a firstswitching element, which is used for applying an input voltage to aninductive storage element, and of a second switching element, which isused as a first rectifier element in a rectifier arrangement, in astep-up converter. The method involves the following processes during anactuation cycle: turning on the first switching element for a firstturned-on period, during which the second switching element is off;turning on the second switching element at the end of the firstturned-on period or after the end of the first turned-on period for asecond turned-on period; monitoring a current which flows through thesecond switching element or the inductive storage element during thesecond turned-on period, and terminating the second turned-on periodfollowing a change in the current direction of this current from a firstcurrent direction to an opposite second current direction.

Another embodiment relates to a step-up converter which has: inputterminals for applying an input voltage; output terminals for providingan output voltage; an inductive storage element which is coupled to theinput terminals; a first switching element, which is connected up forapplying the input voltage to the inductive storage element; a rectifierarrangement having a first capacitive storage element and a secondswitching element, which rectifier arrangement is connected between theinductive storage element and the output terminals; a second capacitivestorage element, which is connected between the input terminals; anactuating circuit for providing a first actuation signal for the firstswitching element and a second actuation signal for the second switchingelement.

Another embodiment relates to an actuating circuit for generating afirst actuation signal for a first switching element, which is used forapplying an input voltage to an inductive storage element, and a secondactuation signal for a second switching element, which is used as afirst rectifier element in a rectifier arrangement, in a step-upconverter. The actuating circuit is designed so that during an actuationcycle it generates a turn-on level for the first actuation signal for afirst turned-on period and generates a turn-off level for the secondactuation signal during this first turned-on period; generates a turn-onlevel for the second actuation signal at the end of the first turned-onperiod or after the end of the first turned-on period for a secondturned-on period; monitors a current flowing through the secondswitching element or the inductive storage element during the secondturned-on period and terminates the second turned-on period after achange in the current direction of this current from a first currentdirection to an opposite second current direction has been detected.

FIG. 1 illustrates one embodiment of a step-up converter (boostconverter) using an electrical equivalent circuit diagram. The step-upconverter has input terminals 11, 12 for applying an input voltage Vinand output terminals 13, 14 for providing an output voltage Vout. In theembodiment illustrated, one 12 of the input terminals and one 14 of theoutput terminals are connected to a common electrical potential, such asground. By way of example, the output voltage V_(out) is used forsupplying power to a load Z which can be connected to the outputterminals 13, 14 (illustrated in dashes). In addition, the step-upconverter has an inductive storage element 22 and a first switchingelement 21 for applying the input voltage Vin to the inductive storageelement 22. In the example shown, the first switching element 21 isconnected in series with the inductive storage element 22, wherein aseries circuit containing the inductive storage element 22 and the firstswitching element 21 is connected between the input terminals 11, 12.

The inductive storage element 22 may be any inductive storage elementfor the inductive or magnetic storage of energy. The inductive storageelement 22 may be a storage inductor having an inductor core (notillustrated). The first switching element 21 has a load path, which isconnected in series with the inductive storage element 22, and a controlconnection for supplying a first actuation signal S1. In the embodimentillustrated, this switching element is in the form of a MOSFET. Adrain-source path D-S in this MOSFET forms the load path, while a gateconnection G in this MOSFET forms the control connection. In thisembodiment, it should be pointed out that instead of a MOSFET it isnaturally also possible to use any other switching elements,particularly semiconductor switching elements, such as IGBTs or bipolartransistors.

The first switching element 21 has a parasitic capacitance, which isillustrated in FIG. 1 and is denoted by the reference symbol 23, betweenits load path connections. In a MOSFET, this parasitic capacitance 23 isdetermined by the MOSFET's drain-source capacitance, but it is alsodependent on the MOSFET's gate-drain capacitance.

The step-up converter has a rectifier arrangement 30 which is connectedbetween the inductive storage element 22 and the output terminals 13,14. In the embodiment illustrated, this rectifier arrangement 30 has afirst capacitive storage element 33 which is connected between theoutput terminals 13, 14 and provides the output voltage Vout. Thiscapacitive storage element 33 is in the form of a capacitor, forexample, and is subsequently also referred to as the output capacitor ofthe step-up converter. The inductive storage element 22 and the outputcapacitor 33 have a second switching element 31 connected between themin the rectifier arrangement 30. This second switching element 31 has aload path connected between the inductive storage element 22 and theoutput capacitor 33 and has a control connection for supplying a secondactuation signal S2. In the embodiment illustrated, the second switchingelement 31 is in the form of a MOSFET whose drain-source path forms theload path and whose gate connection forms the control connection. Inthis embodiment, it should be pointed out that instead of a MOSFET it isnaturally possible to use any other switching elements, particularlysemiconductor switching elements, such as IGBTs or bipolar transistors.

In the rectifier arrangement 30, the second switching element 31performs the function of a first rectifier element when it is actuatedin suitable fashion; such suitable actuation will be explained below. Asecond rectifier element 32, for example, a diode 32, may be connectedin parallel with the load path of the second switching element 31. Ifthe second switching element 31 is in the form of a MOSFET, this diode32 may be the integrated body diode of the MOSFET. In the case of ann-channel MOSFET, this body diode is located in the forward directionbetween the source and drain of the MOSFET. In one embodiment, thesecond rectifier element 32 may be in the form of a separate diode,particularly in the form of a Schottky diode, which is connected inparallel with the load path of the switching element 31. When a MOSFETwith an integrated body diode is used as the second switching element,this separate diode is then connected in parallel with the integratedbody diode of the MOSFET.

For the purpose of the generating the first and second actuation signalsS1, S2 for the first and second switching elements 21, 31, an actuatingcircuit 40 is provided, which is illustrated only schematically as acircuit block in FIG. 1. The way in which this actuating circuit 20works will become clear from the following explanation of one embodimentfor actuating the first and second switching elements 21, 31 or forgenerating the actuation signals S1, S2 for these first and secondswitching elements 21, 31. Such a method will be explained withreference to FIG. 2, which illustrates time profiles for the firstactuation signal S1, the second actuation signal S2, a current I22through the inductive storage element 22 and a voltage V21 across thefirst switching element 21.

In the embodiment illustrated, the actuation signals S1, S2 are binarysignals which can assume a turn-on level or a turn-off level. When therespective actuation signal is at a turn-on level, the switching elementactuated by the actuation signal is on, and when the respectiveactuation signal is at a turned-off level, the switching elementactuated by the actuation signal is off. For the purposes of theexplanation, it will subsequently be assumed that an upper signal level(high level) of the respective actuation signal corresponds to a turn-onlevel and that a lower signal level (low level) of the respectiveactuation signal corresponds to a turn-off level.

The first and second switching elements 21, 31 are turned on and offcyclically, i.e. in basically the same way during successive actuationcycles. In this embodiment, during an actuation cycle, the firstswitching element 21 is turned on for a first turned-on period Ton1 andthe second switching element 31 is turned on for a second turned-onperiod Ton2. The two switching elements 21, 31 are turned on atdifferent times in order to avoid shorting the output capacitor 33. Inone embodiment, an actuation cycle extends from when the first switchingelement 21 is turned on at the start of a first turned-on period Ton1 towhen the first switching element 21 is turned on again at the start of asubsequent first turned-on period. In the embodiment illustrated in FIG.2, such an actuation cycle extends from a time t1 to a time t7. Thelength of the turned-on cycle is Tc in this instance. If the first andsecond switching elements 21, 31 have a switching frequency fs, fs isequal to 1/Tc. This switching frequency is in the region of 100 kHz orabove, for example.

In the case of the method explained with reference to FIG. 2, anactuation cycle can be divided into six different phases, which aresubsequently denoted by A to F.

A first actuation phase A corresponds to the first turned-on periodTon1, during which the first switching element 21 has been turned on bythe first actuation signal S1. The second switching element 31 is offduring this first actuation phase A. Ignoring the voltage drop acrossthe first switching element 21, which is on, approximately the entireinput voltage Vin is thus across the inductive storage element 22 duringthe first actuation phase A. The current 122 through the inductivestorage element 22 then rises. For a change dI22/dt in this current I22over time, the following is then true:

$\begin{matrix}{\frac{{\mathbb{d}I}\; 22}{\mathbb{d}t} = {\frac{Vin}{L}.}} & (1)\end{matrix}$

In this embodiment, Vin denotes the input voltage and L denotes theinductance of the inductive storage element 22. The slope of the currentI22 is therefore proportional to the input voltage Vin and inverselyproportional to the inductance of the inductive storage element 22. Thisrelationship applies when the inductive storage element 22 does notbecome saturated or is not magnetized to the point of saturation duringthe first turned-on period Ton1, which is assumed from the explanationbelow. If the inductive storage element 22 were magnetized to the pointof saturation, the current 122 would not—as illustrated—rise linearly.The present method also applies for those first turned-on periods Ton1in which the inductive storage element becomes saturated; for theexplanation which follows, however, linear time profiles are assumed.

The end of the first turned-on period Ton1 is reached at a time t2. Atthis time t2, a second actuation phase B begins, during which bothswitching elements 21, 31 are off. The current I22 flowing through theinductive storage element 22 is accepted by the second rectifier element32, which is connected in parallel with the second switching element 31,during this second actuation phase B. The second actuation phase B endsat a time t3, from which the second switching element 22 is turned onfor a second turned-on period Ton2 by using the second actuation signalS2. From this time, the current 122 flows through the second switchingelement 22 which, when in the on state, has lower on-state power lossesthan the second rectifier element 32—which is in the form of a diode,for example—or which has the same on-state power losses. The use of thesecond switching element 31 as a rectifier element serves to reduce thepower loss arising in the step-up converter in comparison with the useof a conventional diode.

The first and/or the second switching element 21, 31 may be in the formof MOSFETs, in particular, which operate on the basis of thecompensation principle. Such components, which are also calledsuperjunction MOSFETs, basically have a low area-specific turn-onresistance, and hence low power loss. The power loss which arises in aMOSFET which is on, particularly a MOSFET operating on the basis of thecompensation principle, is lower than the power loss which arises in aforward-biased diode which has the same dielectric strength as theMOSFET.

The second actuation phase B, during which both switching elements 21,31 are off, is present merely for safety reasons and ensures that thefirst switching element 21 is safely off when the second switchingelement 31 is turned on. The length of this second actuation phase B ischosen, taking account of customary time delays for the first and secondswitching elements 21, 31, such that the first switching element 21 issafely off and the second switching element 31 is turned on.

At the end of the first turned-on period Ton1, that is to say when thefirst switching element 21 is disconnected, the voltage V21 across theload path of the first switching element 21 starts to rise. This voltagethen rises up to a value which corresponds to the sum of the outputvoltage Vout and the voltage across the second switching element 31 orthe second rectifier element 32. In this context, it should be notedthat when a MOSFET with an integrated freewheeling diode is used as thefirst switching element 21, this MOSFET naturally needs to be connectedup such that it is able to block such a voltage. When an n-channelMOSFET is used as the first switching element 21, the drain connectionof this MOSFET needs to be connected to the inductive storage element22.

In this embodiment, the length of the second actuation phase B can bechosen, on the basis of the time profile of the voltage V21, such thatthis actuation phase B ends after a delay time ΔT has elapsed, forexample, after the voltage V21 has reached its maximum value or afterthis voltage V21 has reached a prescribed limit value. The rise in thisvoltage to the maximum value serves as an indication of the first switchS21 being off. The delay time ΔT is used for safety.

The by using the second switching element 31, which is on, may benegligible in comparison with the output voltage Vout, for example whenthe step-up converter are used to generate what is known as anintermediate-circuit voltage from a mains input voltage. Customaryintermediate-circuit voltages for this situation are in the region ofseveral 100 V, e.g., 400 V. By way of example, the voltage drop across apower MOSFET which is on is in the range from a few tenths of a volt toa maximum of a few volts and is negligible in comparison.

When the first switching element 21 is turned off at the end of thefirst actuation phase A, the current 122 through the inductive storageelement 22 begins to fall. Provided that the inductive storage element22 has not previously been magnetized as far as the range of saturation,the current 122 falls linearly. For a change dI22/dt in the current I22over time, the following is true:

$\begin{matrix}{\frac{{\mathbb{d}I}\; 22}{\mathbb{d}t} = {\frac{{Vin} - {Vout}}{L}.}} & (2)\end{matrix}$

A third actuation phase C starts at a time t3, at which the secondswitching element 31 is turned on by using the second actuation signalS2. During this third actuation phase C, the current I22 through theinductive storage element 22 flows further in the direction illustratedin FIG. 1, i.e. from the inductive storage element 22 via the secondswitching element 31 to the output capacitor 33 or the load Z. Thiscurrent direction is subsequently referred to as the first currentdirection. The amplitude of the current I22 decreases in this embodimenton the basis of equation (2). The end of this third actuation phase C isreached at a time t4, at which the current through the inductive storageelement 22 has fallen to zero, that is to say at which the inductivestorage element 22 is fully demagnetized. The second switching element31 remains on beyond this time t4. The current I22 through the inductivestorage element 22 thereby changes its direction from the first currentdirection to an opposite second current direction, which is subsequentlyalso referred to as the reverse direction. A current flowing through theinductive storage element in this direction is subsequently alsoreferred to as a reverse current. This reverse current is supplied bythe output capacitor 33.

At the time t4, at which the inductive storage element 22 is fullydemagnetized, or at which the current I22 has its current directionreversed, a fourth actuation phase D starts. During this actuation phaseD, electrical energy is stored in the inductive storage element 22, orthe inductive storage element 22 is magnetized. This energy comes fromthe output capacitor 33. For a change in the current I22, therelationship according to equation (2) also applies during this fourthactuation phase D. Unlike in the third actuation phase C, the amplitudeof the current I22 increases during this fourth actuation phase D,during which the current flows in the opposite direction in comparisonwith the third actuation phase D. During this fourth actuation phase D,a voltage across the first switching element 21 corresponds to thedifference between the output voltage Vout and the voltage drop acrossthe second switching element 31, which is on. The voltage V21 across thefirst switching element 21 changes from the third to the fourthactuation phase C, D merely by a value which is twice as high as thevoltage drop across the second switching element 31, which is on, if itis assumed that the voltage drop is the same in both directions.Ignoring the voltage drop across the first switching element 21, whichis on, in comparison with the output voltage Vout, there is nosignificant change in the voltage V21 across the first switching element21 between the third and fourth actuation phases C, D, which has beenassumed in the illustration illustrated in FIG. 2.

A flow of current through the inductive storage element 22 in thereverse direction is made possible by a second capacitive storageelement 24, which is connected between the input terminals 11, 12. Asecond capacitive storage element 24 of this kind is required in casesin which it is not possible to feed a current back into the voltagesource (not illustrated) which provides the input voltage Vin. By way ofexample, such current feedback is not possible when the input voltageVin is provided by using a bridge rectifier 100 from an AC voltage Vn,for example a mains voltage, in a manner which is fundamentally known.The use of the second capacitive storage element 24 can be dispensedwith if it is possible to feed a current back into the voltage source,or if the bridge rectifier 100 contains capacitive storage elementswhich are connected in parallel with the rectifier elements which thebridge rectifier contains.

To simplify illustration, the profile of the current I22 through theinductive storage element during the activation phase D is illustratedas a linear current profile. A linear current profile of this kindpresupposes that the input capacitor 24 is not charged beyond the valueof the input voltage Vin during the actuation phase D, so that thevoltage across the inductive storage element 22 continues to correspondto the difference Vout-Vin between the output and the input voltage. Inone embodiment, the capacitance of the input capacitor 24, for example,is of sufficient magnitude, taking into account the charge flowing tothe input capacitor 24 during the phase D, for the voltage across theinput capacitor 24 not to rise above the value of the input voltage Vin.Aside from that, although a rise in the voltage Vin during the phase Dwould result in a nonlinear profile for the current I22 during thisphase, the fundamental way in which the method works, which has beenexplained, does not change as a result.

The fourth actuation phase D ends at a time t5, at which the secondswitching element 31 is turned off. In this embodiment, the firstswitching element 21 continues to remain off at first. At the start of afifth actuation phase E, beginning at this time t5, the voltage V21across the first switching element 21 first of all corresponds to thevoltage during the fourth actuation phase D, that is to sayapproximately the output voltage Vout. This is dependent upon theparasitic capacitance 23, which has also been charged to this voltage atthe time t5. During the fifth actuation phase E, the parasiticcapacitance 23 is discharged by the current I22 which continues to flowin the reverse direction, so that the voltage V21 across the firstswitching element 21 falls to zero. In FIG. 2, t6 denotes a time atwhich the parasitic capacitance 23 has been discharged completely, orthe voltage V21 across the first switching element 21 has fallen tozero. When the parasitic capacitance 23 has been discharged, a flow ofcurrent through the inductive storage element 22 in the reversedirection is made possible by a freewheeling element 25 which isconnected in parallel with the load path of the first switching element21. In the embodiment illustrated in FIG. 1, this freewheeling elementis in the form of a diode. When a MOSFET is used as the first switchingelement 21, this diode 25 may be formed by the integrated body diode ofthe MOSFET.

During a sixth actuation phase F, which starts at the time t6 and duringwhich the freewheeling element 25 accepts the current 122 from theinductive storage element 22, the voltage V21 across the first switchingelement 21 corresponds to the on-state voltage of the freewheelingelement, which is approximately negligible in comparison with the outputvoltage Vout which was previously across the switching element 21,however. During the sixth actuation phase F, the polarity of the voltageacross the first switching element 21 is opposite to the polarity duringthe other actuation phases.

The sixth actuation phase F, and hence the actuation cycle, ends at atime t7, at which the first switching element 21 is turned on again fora first turned-on period Ton1. The sixth actuation phase F, for exampleno later than when the current 122 through the inductive storage element22 has fallen to zero. In the embodiment illustrated in FIG. 2, a timet7, at which the sixth actuation phase F ends, corresponds to this“zero-current time”. If the sixth actuation phase F were to end later,free transformer oscillations could arise after the zero-voltage time,and these can be undesirable. In general, the time t7, at which thesixth actuation phase F ends and at which the first switching element isturned on again, can be chosen such that it is situated between the timet6 and the “zero-current time”. By way of example, this time isascertained using a limit value with which the current I22 is compared.The first switch S21 is turned on again whenever the magnitude of thecurrent I22 has fallen to this prescribed threshold value, for example,the threshold value being able to be zero or not equal to zero.

During the fifth and sixth actuation phases E, F, the current I22continues to flow in the reverse direction through the inductive storageelement 22. An amplitude for this current decreases during theseactuation phases, however.

To simplify illustration, the time profile of the current I22 in figureduring the actuation phases E and F is illustrated as a linear timeprofile. In fact, the current does not have a linear profile during thefifth actuation phase E; however, the magnitude of the current I22increases steadily during this phase—as illustrated. During the sixthphase F, the decrease in the current I22 has an approximately linearprofile over time, with dI22/dt^(˜)Vin/L.

The method explained allows the first switching element 21 to be turnedon at a time (t7 in FIG. 2) at which the voltage across this switchingelement 21 has fallen significantly in comparison with a voltage whichis present at a time at which the inductive storage element 22 has beendemagnetized for the first time (t4 in FIG. 2). In comparison withconventional methods, in which a switching element which is used forconnecting an inductive storage element to an input voltage which was tobe turned on again as soon as the inductive storage element has beendemagnetized, the switching losses can be significantly reduced in thecase of the method explained. If, with reference to FIGS. 1 and 2, thefirst switching element 21 were to be turned on again as early as thetime t4 at which the inductive storage element has been demagnetized,the electrical energy previously stored in the parasitic capacitance 23would be converted to heat in the first switching element 21, i.e. wouldcontribute to the power loss. In the case of the method explained, theelectrical energy stored in the parasitic capacitance 23 is by contrastfed back to the voltage source via the inductive storage element 22 oris buffer-stored in the input capacitor 24 until the first switchingelement 21 is next turned on.

In addition, the method explained avoids hard switching edges in avoltage V21 across the first switching element 21, since the firstswitching element 21 is not turned on until a time at which this voltageV21 has fallen to the value of the on-state voltage of the freewheelingelement 25. If the first switching element were to be turned on again asearly as the demagnetization time t4, this voltage V21 would fall veryquickly from a high voltage value, which is in the region of the outputvoltage, to a low voltage value, which corresponds to the voltage dropacross the component which is on. This could result in EMC problems. Inthis embodiment, it should be noted that the switching, particularly thedisconnection, of the second switching element is rather uncritical interms of EMC problems in the method explained, since the voltage acrossthe second switching element 31 upon disconnection at the end of phase Dis small.

The method explained ensures complete discharge of the parasiticcapacitance 23 of the first switching element 21 particularly when theenergy stored in the inductive storage element 22 during the fourthactuation phase D corresponds, at a time t5, when the reverse currentI22 reaches its maximum value, at least to the electrical energy storedin the parasitic capacitance 23 at this time t5, that is to say when thefollowing is true:

$\begin{matrix}{{\frac{1}{2} \cdot L \cdot {Ipn}^{2}} \approx {\frac{1}{2} \cdot C_{23} \cdot {{Vout}^{2}.}}} & (3)\end{matrix}$

In this embodiment, Ipn denotes the maximum amplitude of the current I22during the fourth actuation phase D, which is subsequently also referredto as the peak reverse recovery current, L denotes the inductance of theinductive storage element, C₂₃ denotes the capacitance value of theparasitic capacitance 23 and Vout denotes the output voltage. Theestimate made on the basis of equation (3) in this instance is based onthe assumption that the voltage across the first switching element 21during the fourth actuation phase D corresponds approximately to theoutput voltage Vout; the energy stored in the parasitic capacitance 23at a time t5 therefore corresponds approximately to the term indicatedon the right in equation (3).

The relationship explained with reference to equation (3) is based onthe simplistic assumption that the capacitance value C₂₃ of theparasitic capacitance 23 is independent of the voltage across the firstsemiconductor switching element 21 and that the capacitance value of aparasitic capacitance 34 of the second semiconductor switching element31, which capacitance is present in parallel with the load path of thesecond semiconductor switching element 31, is zero. As alreadyexplained, the two semiconductor switching elements 21, 31 may be in theform of compensating components, in particular. The parasiticcapacitance or output capacitance of such compensating components isdependent on the voltage across the load path of the component anddecreases as the voltage increases.

FIG. 10 schematically illustrates the profile of the capacitance valueC_(oss) of an effective capacitance which is active at the circuit nodewhich is common to the first and second semiconductor switching elements21, 31. This effective capacitance is made up of the capacitance valuesof the voltage-dependent output capacitances 23, 31 of the first andsecond semiconductor switching elements 21, 31, and in FIG. 10 isplotted as a function of the voltage V21 across the first semiconductorswitching element 21. At small voltage values for this voltage V21, thecapacitance value of this capacitance is definitively determined by theoutput capacitance 23 of the first semiconductor switching element 21,which output capacitance has its maximum value at a voltage V21 of zero.Accordingly, the capacitance value of this effective capacitance atlarge voltage values of the voltage V21 is definitively determined bythe output capacitance 34 of the second semiconductor switching element31. This output capacitance assumes its maximum value when the voltageacross the second semiconductor switching element 31 is zero, that is tosay when the voltage V21 across the first semiconductor switchingelement 21 corresponds to the output voltage Vout. A minimum for thecapacitance value of this effective capacitance is at a voltage V21between zero and the output voltage Vout.

When considering output capacitances 23, 34 with voltage-dependentcapacitance values, the electrical energy E_(ind) stored in theinductive storage element 22 during the flow of current in the reversedirection must satisfy the following condition:

$\begin{matrix}{E_{ind} = {\int_{0}^{Vout}{{C_{oss} \cdot U}{\mathbb{d}U}}}} & \left( {3a} \right)\end{matrix}$in this embodiment, C_(oss) denotes the voltage-dependent effectivecapacitance.

To ensure complete discharge of the parasitic capacitance 23, oneembodiment has provision for calculation of a minimum necessary peakreverse recovery current value for complete discharge of the parasiticcapacitance 23, which is subsequently referred to as Ipn_(min), and forthe second turned-on period Ton2 to be respectively terminated when thecurrent I22 flowing in the second current direction reaches this peakvalue Ipn_(min), that is to say when Ipn_(min)=I22 is true. Withreference to equation (3), the following is approximately true for thisminimum peak reverse recovery current value or threshold value Ipn ofthe current I22, at which the second switching element 31 is supposed tobe disconnected:

$\begin{matrix}{{Ipn}_{\min} \approx {\sqrt{\frac{C_{23}}{L}} \cdot {{Vout}.}}} & (4)\end{matrix}$

When compensating components are used as switching elements, thisequation needs to be adapted as appropriate taking into account equation(3a).

This peak reverse recovery current value Ipn_(min) is thereforedependent on the capacitance value C₂₃ of the parasitic capacitance 23and the inductance L of the inductive storage element 22, which arefixed variables, and on the output voltage Vout. The output voltage Voutis a variable which is controlled by the step-up converter and which issupposed to be kept at least approximately constant regardless of aconnected load Z. The minimum peak reverse recovery current valueIpn_(min) may therefore be stored as a constant variable in theactuating circuit 40. In this embodiment, the actuating circuit 40 isdesigned to turn off the second semiconductor switching element 31 eachtime the current I22 in the reverse direction has respectively risen tothis threshold value Ipn_(min). It should be noted that the peak reverserecovery current value calculated on the basis of equation (4)represents the minimum value which the reverse current should reach inorder to achieve complete discharge of the parasitic capacitance. Itgoes without saying that the second switching element 31 may also bedisconnected only when the reverse current has risen above the valuebased on equation (4). In the text below, Ipn generally denotes the peakreverse recovery current value, at which the second switching element isdisconnected. To achieve complete discharge, this value should begreater than or equal to the value Ipn_(min) defined in equation (4).

In one embodiment, it is possible to store information about thecapacitance value C₂₃ of the parasitic capacitance 23 and the inductancevalue L of the inductive storage element 22 in the actuating circuit 40and to measure the output voltage Vout or the voltage V21 across thesemiconductor switching element 21 and to calculate the threshold valueIpn in the actuating circuit 40 in line with equation (4). The actuatingcircuit 40 may be programmable (in a manner which is not illustrated inmore detail), so that the information required for calculating thethreshold value Ipn can be programmed externally. This allows use of theactuating circuit 40 in step-up converters with different dimensionalspecifications, i.e. with different inductances and parasiticcapacitances.

In the method explained, regulation of the output voltage Vout to avalue which is at least approximately independent of a power consumptionby the load Z is effected using the first turned-on period Ton1 of thefirst switching element 21, for example. The step-up converter's powerconsumed on average via the input terminals 11, 12 needs to be matchedto the output power consumed by the load Z via the output terminals 13,14, in order to keep the output voltage Vout constant. The average powerconsumption of the step-up converter is directly dependent on the firstturned-on period Ton1, which means that the output voltage Vout can beregulated to a constant value using the first turned-on period Ton1. Ifthe load Z has a high power consumption, the first turned-on period Ton1is set to a high value in order to keep the output voltage Voutconstant. If the load Z has only a low power consumption, the firstturned-on period Ton1 is set to a low value in order to keep the outputvoltage Vout constant. To adjust this first turned-on period Ton1, acontrol signal is provided which is dependent on a power consumption bythe load at present or on a time profile of the output voltage Vout inthe past. This control signal is used to set the first turned-on periodTon1.

FIG. 3 illustrates one embodiment for generating such a control signalRS. In the embodiment illustrated, the control signal RS is available atthe output of a controller 43, which may have a proportional controlresponse (P response), an integral control response (I response) or aproportional integral response (PI response). This controller 43 issupplied with a measurement signal Sv_(out) which is proportional to theoutput voltage Vout and which—as illustrated—is generated from theoutput voltage Vout using a voltage divider 41, 42, for example. Thecontroller 43 is also supplied with a reference signal S_(REF) whichrepresents a setpoint variable to which the output voltage Vout needs tobe regulated. The controller 43 uses the measurement signal S_(vout) andthe reference signal S_(REF) to ascertain a control error and generatethe control signal RS taking this control error into account. Thecontrol signal RS represents—depending on the control response of thecontroller 43—a present and/or past error in the output voltage Voutfrom the setpoint value represented by the reference signal S_(REF) andthus represents a power consumption by the load Z. By way of example,this control signal RS is generated such that its value increases as thepower consumption of the load Z increases.

The controller 43 may be any controller which has the desired controlresponse. In particular, the controller 43 may be an analog controlleror else a digital controller. The control signal RS available at theoutput of the controller 43 is supplied to a signal generation circuit50 which, like the controller 43, is part of the actuating circuit 40and which generates the first and second actuation signals S1, S2.

An example of such a signal generation circuit 50 is illustrated in FIG.4. This signal generation circuit has first and second flipflops 51, 52which are in the form of RS-type flipflops in the embodiment illustratedand which each have a Set input S, a Reset input R and a noninvertingoutput Q. The output Q of the first flipflop 51 provides the firstactuation signal S1, and the output of the second flipflop 52 providesthe second actuation signal S2. Optionally, the flipflops 51, 52 havedownstream driver circuits 58, 59 which serve to convert signal levelsfor the logic signals available at the outputs of the flipflops 51, 52to signal levels which are suitable for actuating the first and secondswitching elements 21, 31. For the purposes of explanation, it will beassumed that turn-on levels of the actuation signals S1, S2 are presenteach time the flipflops 51, 52 have been set, and that turn-off levelsof the actuation signals S1, S2 are present when the flipflops 51, 52have been reset.

To set the first flipflop 51, and hence to stipulate the start of afirst turn-on period Ton1, a first evaluation circuit 52 is providedwhich, in the example, is supplied with a current measurement signalS_(I22). This current measurement signal S_(I22) is dependent on thecurrent I22 flowing through the inductive storage element 22. Inparticular, this current measurement signal S_(I22) is proportional tothis current and can be generated by using any current measuringarrangement, which is not illustrated in more detail in FIG. 1. Thefirst evaluation circuit 53 is designed to evaluate this currentmeasurement signal S_(I22) and to set the flipflop 51 if a turn-oncriterion for the first switching element 21 is met. With reference toFIG. 2, this turn-on criterion is met, for example, when the current I22has reached a prescribed turn-on threshold value. A signal for resettingthe first flipflop 51, and hence for terminating the first turned-onperiod Ton1, is generated by a comparator 56 in the embodimentillustrated, the comparator comparing a signal generated by atriangular-waveform signal generator or sawtooth signal generator 55with the control signal RS. The signal generator 55 is actuated by thefirst evaluation circuit 53 and generates a triangular-waveform signaleach time the first flipflop 51 is set. The flipflop 51 is reset eachtime the triangular-waveform signal has risen to the value of thecontrol signal RS. This period of time directly determines the firstturned-on period Ton1 and, in the embodiment illustrated, is longer thelarger the control signal RS.

In the signal generation circuit 50 illustrated, a turn-on level for thesecond actuation signal S2 is generated when a delay time has elapsedafter the first flipflop 51 has been reset. In one embodiment, theoutput signal from the comparator 56 is supplied to the Set input S ofthe second flipflop 52 via a delay element 57. In this embodiment, adelay time prescribed by the delay element 57 determines the time delaybetween resetting the first flipflop, i.e. the end of the firstturned-on period Ton1, and setting the second flipflop 52, i.e. thestart of the second turn-on period Ton2.

To reset the second flipflop 52, i.e. to terminate the second turned-onperiod Ton2, a second evaluation circuit 54 is provided which issupplied with the current measurement signal S_(I22) and with thethreshold value Ipn. In this embodiment, the second flipflop 52 is resetby the second evaluation circuit 54 each time the current measurementsignal S_(I22) indicates that the current I22 is flowing through theinductive storage element 22 in the second current direction and hasreached the threshold value Ipn.

It should be pointed out that instead of a single current measurementsignal S_(I22) it would also be possible for two current measurementsignals to be provided, namely a first current measurement signal, whichrepresents the current in the first switching element 21 of thefreewheeling element 25 connected in parallel therewith and which issupplied to the first output value circuit 53, and a second currentmeasurement signal, which represents the current through the secondswitching element 31 and which is supplied to the second evaluationcircuit 54. It should also be pointed out that FIG. 4 is meant toexplain only the fundamental manner of operation of an example of thesignal generation circuit 50. It goes without saying that the circuitcomponents illustrated in FIG. 4 may be in the form of analog or digitalcircuit components.

Instead of ascertaining the disconnection time (t5 in FIG. 2) of thesecond switching element 31 by comparing the current I22 with athreshold value which corresponds to the desired peak reverse recoverycurrent value, a further example has provision for the disconnectiontime t5 to be stipulated by taking the disconnection time t2 of thefirst switching element 21 as a basis for calculating the period of timewithin which the current direction of the current I22 is reversed andwithin which the amplitude of the current I22 in the second currentdirection reaches the desired peak reverse recovery current value Ipn.With reference to FIG. 2, this period of time corresponds to the sum ofthe delay time ΔT and the second turned-on period Ton2 or the period oftime between the times t2 and t5. The following is true for this periodof time:

$\begin{matrix}{{Tdel} = {\frac{{{\frac{Vin}{L} \cdot {Ton}}\; 1} + {Ipn}}{\frac{{Vout} - {Vin}}{L}} = {{\frac{Vin}{{Vout} - {Vin}}{Ton}\; 1} + {\frac{{Ipn} \cdot L}{{Vout} - {Vin}}.}}}} & (5)\end{matrix}$

In this embodiment, Tdel denotes the delay time between the firstswitching element 21 being turned off and the second switching element31 being turned off later. With reference to FIG. 5, this delay timeTdel can be calculated from the input voltage Vin, the output voltageVout, the first turned-on period Ton1 and the desired peak reverserecovery current value Ipn of the current I22.

An example of a signal generation circuit 50 which actuates the firstand second switching elements 21, 31 in this manner is illustrated inFIG. 5. This signal generation circuit 50 differs from the oneillustrated in FIG. 4 in that a second evaluation circuit 60, whichresets the second flipflop 52, is supplied with the reset signal fromthe first flipflop 51. This reset signal determines the disconnectiontime t2 of the first switching element 21. The second evaluation circuit60 takes account of equation (5) to calculate the delay time Tdel beforethe second switching element 31 is turned off from this time. In oneembodiment, the second evaluation circuit 60 is supplied with an inputvoltage signal S_(Vin), an output voltage signal S_(Vout), the peakvalue signal Ipn and a turned-on period signal S_(Ton1). The inputvoltage signal S_(Vin) contains information about the input voltage Vin,the output voltage signal S_(Vout) contains information about the outputvoltage Vout, and the first turned-on period signal S_(Ton1) containsinformation about the first turned-on period Ton1. The output voltagesignal S_(Vout) can be ascertained by measuring the output voltage Vout,this not being illustrated in more detail.

Taking account of the fact that the output voltage Vout is alwaysadjusted to a setpoint value, and hence is at least approximatelyconstant, the output voltage signal S_(Vout) may also be permanentlystored in the actuating circuit or the signal generation circuit 50.Information about the inductance L, which is likewise needed forcalculating the delay time Tdel, is permanently stored in the secondevaluation circuit 60, for example. The second evaluation circuit 60 maybe programmable in this instance (this is not illustrated in moredetail), which means that the information about the inductance L can beset or programmed externally.

The input voltage Vin can be measured (not illustrated in more detail)in order to ascertain the input voltage signal S_(Vin). In addition, itis also possible to ascertain the input voltage Vin by measuring thecurrent I22 at two different times during the first turned-on periodTon1 and calculating the input voltage Vin using knowledge of theinductance and the time interval between the two measurement points. Inthis context, the following is true:Vin=(I22(t12)−I22(t11))−L  (6).

In this embodiment, I22(t12) and I22(t11) denote the value of thecurrent I22 at the measurement times t12 and t11, the measurement timet12 coming after the measurement time t11. The measurement times areillustrated by way of example in FIG. 2 to aid understanding.

It goes without saying that it is also possible to ascertain the inputvoltage Vin by measuring the current I22 at two measurement times whichare apart in time during the falling edge of the current, that is to sayduring the second turned-on period Ton2.

The step-up converter explained is particularly suitable for convertingan input voltage Vin into an output voltage Vout which is a rectifiedmains voltage Vn. In the case of a sinusoidal mains voltage Vn at afrequency of 50 Hz or 60 Hz, the input voltage Vin is a voltage in theform of the magnitude of a sine wave at a frequency of 100 or 120 Hz. Inthis embodiment, the input voltage Vin varies over time. However, thefrequency of the input voltage Vin is low in comparison with switchingfrequencies at which the first and second switching elements 21, 31 areactuated. By way of example, these switching frequencies are in theregion of 100 kHz, and above. In particular, the switching frequenciesare in the range between 100 kHz and 250 kHz, for example. The inputvoltage Vin may be assumed to be approximately constant in thisembodiment, at least for the duration of an actuation cycle, so that theprevious statements are also valid for an input voltage Vin which resultfrom a mains AC voltage Vn through rectification.

Step-up converters which are used for converting an input voltage Vinresulting from a mains voltage Vn in many cases have the requirementthat an average current consumption by the step-up converter needs to beproportional to the input voltage Vin. The power consumption is thenproportional to the square of the input voltage. This reduces reactivepower absorption from the mains. Step-up converters which ensure suchproportionality between the input voltage Vin and the average value ofthe input current are also called power factor correction circuits(power factor controllers, PFC). It is basic knowledge that suchproportionality between the input voltage and the average value of aninput current, which is denoted by Iin in FIG. 1, can be achieved for astep-up converter by virtue of the switching element which connects theinductive storage element to the input voltage being turned on forconstant turned-on periods in each case, and is turned on again as soonas the inductive storage element is completely demagnetized. In the caseof step-up converters in which it is not turned on again immediatelyafter complete demagnetization of the inductive storage element, suchproportionality between the input voltage and the average value of theinput current can be achieved by virtue of a waiting time betweencomplete demagnetization of the inductive storage element and theswitching element's being turned on again being proportional to theperiod of time between the switching element's being turned on andcomplete demagnetization of the inductive storage element.

In the case of the actuation method explained with reference to FIG. 2,such proportionality between the input voltage Vin and the input currentIin can be achieved by virtue of, with reference to FIG. 2, the periodof time between the time t4 at which the inductive storage element 22 iscompletely demagnetized and the time t7 at which the first switchingelement 21 is turned on again being proportional to the period of timebetween the turn-on time t1 and the time t4. This can be achieved atleast approximately by virtue of the peak reverse recovery current valueInp, at which the disconnection time t5 of the second switching element31 is reached, being set proportionally to the instantaneous value ofthe input voltage Vin, provided that the profile of the current betweenthe times t4 and t7 as well as between the times t1 and t4 is triangularat least approximately or at least over wide ranges of the timeinterval.

In one embodiment, the input voltage Vin can be measured directly, orthe input voltage can be ascertained by measuring the current I22 at twodifferent measurement times taking account of the inductance L of theinductive storage element 22. Setting the threshold value Inp inproportion to the input voltage Vin is a compromise between reducing thepower loss and the power factor correction. This is because with smallinstantaneous values of the input voltage Vin the stored in theinductive storage element 22 during the fourth actuation phase D may notbe sufficient to discharge the capacitive storage element 23 completely,i.e. a peak reverse recovery current value Ipn which is set in thismanner is smaller than Ipn_(min) for small input voltages.

Alternatively, it is possible to set the threshold value Inp for largeinput voltages Vin in proportion to the input voltage Vin and to set toa constant value for small input voltages, the constant value beingchosen such that the energy stored in the inductive storage element 22during the fourth actuation phase D is sufficient to discharge thecapacitive storage element 23 completely. FIG. 6 illustrates an exampleof such dependency of the peak reverse recovery current value Ipn on theinput voltage Vin. In this embodiment, two ranges of values are definedfor the input voltage Vin: a first range of values, which extends from aminimum value Vin_(min), for example zero, for the input voltage Vin toa first limit value Vin₀, and a second range of values, which extendsfrom the first limit value Vin₀ to a maximum possible instantaneousvalue Vin_(max) of the input voltage. For input voltages Vin within thefirst range of values, the peak reverse recovery current value Ipn isfirmly prescribed and, for example, corresponds to the value Ipn_(min)which is at least necessary for complete discharge of the parasiticcapacitance 23. For input voltages Vin within the second range ofvalues, the peak reverse recovery current value Ipn is proportional tothe input voltage Vin. In the case of such a process, the parasiticcapacitance 23 is discharged completely during all fluctuation cycles.Optimum power factor correction is achieved only for input voltageswithin the second range of values, however. In a dash-dot line, FIG. 6also illustrates the dependency of the peak reverse recovery currentvalue on the input voltage Vin for the case in which the peak reverserecovery current value Ipn is proportional to the input voltage for allinstantaneous values of the input voltage.

In another embodiment, provision is made for the peak reverse recoverycurrent value Inp to be set on the basis of the control signal RS andfor the magnitude of the threshold value Inp to be increased if thecontrol signal RS indicates a low power consumption by the connectedload Z. FIG. 7 illustrates the time profile of the current I22 for sucha scenario, in which the threshold value Inp has been shifted towardgreater values in comparison with the embodiment illustrated in FIG. 2.Such variation in the threshold value Inp allows the power consumptionof the step-up converter to be varied given the same first turned-onperiod Ton1. That is to say that when the amplitude of the thresholdvalue Inp is increased the power consumption is reduced given the sameturned-on period Ton1. By increasing the threshold value Inp, it istherefore possible to reduce the power consumption without increasingthe switching frequency. In an extreme case in which the magnitude ofthe threshold value Inp corresponds to the maximum value which thecurrent I22 reaches during the first actuation phase A, the powerconsumption of the step-up converter is even zero on average. Providedthat no bridge rectifier is present, the electrical power consumedpreviously via the input terminals 11, 12 can be fed back into the inputvoltage source completely. Even a negative average power consumption canbe set. In this embodiment, more electrical energy is drained via theinput terminals 11, 12 per actuation cycle than is picked up via them.Over several actuation cycles, this results in the output capacitor 33being discharged.

In this embodiment, the peak reverse recovery current value may havebeen chosen, in particular, so that it assumes the same respectivevalues for individual values in a range of values for the controlsignal, as illustrated in FIG. 8. In the embodiment illustrated in FIG.8, one possible range of values for the control signal has been dividedinto three ranges for which the peak reverse recovery current value Inphas the same respective values, with the magnitude of the peak reverserecovery current value Ipn becoming greater for control signal valueswhich indicate a low power consumption by the load—these are smallcontrol signal values in FIG. 7. Within the individual ranges, the powerconsumption by the load is controlled by the first turned-on periodTon1. This turned-on period, which is illustrated as a dash-dot line inFIG. 8, increases as power consumption increases, i.e. in the example asthe control signal RS becomes larger. Since the power consumption isalso determined by the peak reverse recovery current value in additionto the first turned-on period in the method explained, the same firstturned-on periods Ton1 may be obtained for different ranges of values.Overall, this allows the first turned-on period to be limited to a rangeof values from Ton1 _(min), to Ton1 _(max), which is smaller thanwithout variation of the peak reverse recovery current value Ipn on thebasis of the control signal. In this embodiment, appropriate limiting isobtained for a frequency range of the switching frequency.

In the embodiment illustrated in FIG. 8, the smallest of the possiblepeak reverse recovery current values is chosen, in particular, such thatit is less than or equal to Ipn_(min). If a time-variant input voltageVin is present, the peak reverse recovery current value can be seteither on the basis of the input voltage Vin or on the basis of thecontrol signal.

Referring to the statements made up to this juncture, the duration ofthe sixth actuation phase F, that is to say the period of time betweenthe first V21 falling to zero and the first switch S21 being turned onagain, may be variable, this period of time being set, in particular,such that it ends before the current I22 through the inductive storageelement 22 has fallen to zero. In one example, provision is made for thefrequency and/or the phase of the first and second actuation signals S1,S2 to be set using the duration of this sixth actuation period F.

As explained, the end of the sixth actuation phase F can be ascertainedby comparing the current I22 with a reference value, the end of thesixth actuation period F having been reached in this embodiment when themagnitude of the current I22 has fallen to this reference value, whichis subsequently referred to as I_(F). In the example of a signalgeneration circuit which was explained with reference to FIG. 4, thisevaluation of the current I22 takes place in the first evaluationcircuit.

In this embodiment, the duration of the sixth actuation period F—andhence the frequency and phase of the first and second actuation signalsS1, S2—can be set using this reference value I_(F). By way of example,this reference signal I_(F) is supplied to the actuating circuit 40, asillustrated in dashes in FIG. 1.

In one embodiment, provision is made for the duration of the sixthactuation phase F to be set on the basis of a phase difference betweenone of the two actuation signals S1, S2 and a reference clock signal,specifically with the aim of generating one of the two actuation signalsin phase with the reference clock signal. With reference to FIG. 1, thiscan be achieved, by way of example, by virtue of the reference value IFwhich determines the end of the sixth actuation phase F being generatedby a phase discriminator 60 (illustrated in dashes) which is suppliedwith one of the two actuation signals—in the example the first actuationsignal S1—and with the reference clock signal CLK. In this embodiment,the phase discriminator 60 is part of a control loop and is designed toascertain a phase difference between the reference clock signal CLK andthe actuation signal and to generate the reference value I_(F) on thebasis of this phase difference, with the aim of adjusting the phasedifference between the actuation signal S1 and the reference clocksignal to zero. The phase discriminator 60 is a controller which,depending on its embodiment, generates the reference value I_(F) on thebasis of a present phase difference (P control), on the basis of a pastphase difference (I control) or on the basis of a present and a pastphase difference (PI controller). Such phase discriminators are basicknowledge, which means that there is no need for any furtherexplanations.

Such control of the frequency of one of the two actuation signals maymake sense when reducing electromagnetic interference. In thisembodiment, the reference clock signal can be used to set the spectrumof such interference, so that knowledge of this spectrum can be used totake suitable measures for attenuation. In this connection, it should benoted that setting the frequency and the phase of one of the twoactuation signals also has a direct influence on the frequency and phaseof the other of the two actuation signals.

With reference to FIG. 9, one example has provision for two or moreswitch-mode converters of the type explained above to be connected inparallel. FIG. 9 illustrates two such switch-mode converters by way ofexample, these being denoted by 10 ₁ and 10 _(n). These switch-modeconverters are connected in parallel by virtue of their first inputterminals 11 ₁, 11 _(n) being connected to one another and their secondinput terminals 12 ₁, 12 _(n) being connected to one another and byvirtue of their first output terminals 13 ₁, 13 _(n) being connected toone another and their second output terminals 14 ₁, 14 _(n) beingconnected to one another. In this arrangement, the input terminals aresupplied with the input voltage Vin and the output terminals areconnected to the load Z. The individual switch-mode converters may havea common output capacitance or respective separate output capacitances,this not being illustrated in more detail.

The individual switch-mode converters are actuated, in particular, suchthat their actuation cycles are offset from one another in time, that isto say that the first switches (not illustrated) of the individualswitch-mode converters are actuated at different times from one another.This may make sense both in respect of the power consumption of theoverall arrangement and in respect of the electromagnetic interference:if the first switches are actuated at different times from one another,a more uniform power consumption is achieved by using the input voltageVin than when the first switches are actuated simultaneously. Inaddition, peaks in the energy of the electromagnetic interference areavoided.

For example, the actuation cycles or the actuation of the individualfirst switches can be offset in time by virtue of the individualswitch-mode converters being supplied with reference clock signals CLK₁,CLK_(n), which have the same function as the clock signal explained withreference to figure, that is to say which respectively determine theclock and phase of the individual actuation cycles, and which arephase-shifted relative to one another. It is furthermore possible todetermine one of the switch-mode converters as a master switch-modeconverter and to use this switch-mode converter's first or secondactuation signal to generate a plurality of versions which arephase-shifted relative to one another, which are respectively suppliedto one of the other switch-mode converters as reference clock signals.The phase of the actuation cycles of the master switch-mode convertercan optionally be set by a dedicated reference clock signal.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

1. A method for controlling a converter, including the cyclic actuationof a first switching element for applying an input voltage to aninductive storage element, and a second switching element, used as afirst rectifier element in a rectifier arrangement, comprising: turningon the first switching element for a first turned-on period, duringwhich the second switching element is off; turning on the secondswitching element at the end of the first turned-on period or after theend of the first turned-on period for a second turned-on period;monitoring a current flowing through the second switching element or theinductive storage element during the second turned-on period; andterminating the second turned-on period following a change in currentdirection of the current from a first current direction to an oppositesecond current direction.
 2. The method of claim 1, comprisingterminating the second turned-on period when the current flowing in thesecond current direction reaches a first threshold value.
 3. The methodof claim 2, comprising ascertaining a control signal dependent on theoutput voltage and wherein the first threshold voltage is dependent onthe control signal.
 4. The method of claim 2, comprising wherein thefirst threshold value is dependent on the input voltage.
 5. The methodof claim 1, comprising ascertaining the second turned-on period on thebasis of the input voltage and the first turned-on period.
 6. The methodaccording to claim 1, comprising: ascertaining during the secondturned-on period a change in the current over time; and ascertaining thesecond turned-on period on the basis of the first turned-on period andthe ascertained change over time.
 7. The method of claim 1, comprisinggenerating a control signal dependent on the output voltage, and whereinthe second turned-on period is dependent on the control signal.
 8. Themethod of claim 1, comprising generating a control signal dependent onthe output voltage is generated, and wherein the first turned-on periodis dependent on the control signal.
 9. The method of claim 1, comprisingstarting the first turned-on period when the current through theinductive storage element reaches a prescribed second threshold valueafter the end of the second turned-on period.
 10. The method of claim 9,comprising wherein the second threshold value is zero.
 11. The method ofclaim 1, comprising connecting a second rectifier element in parallelwith the second switching element.
 12. The method of claim 1, comprisingwherein the second switching element is a MOSFET with an integratedfreewheeling diode.
 13. The method of claim 1, comprising wherein thefirst switching element is a MOSFET.
 14. The method of claim 1,comprising: connecting the first switching element in series with theinductive storage element; and connecting a series circuit containingthe inductive storage element and the first switching element betweenthe input terminals; wherein the second switching element is connectedbetween the inductive storage element and a first capacitive storageelement is connected between the output terminals.
 15. A circuitcomprising: an actuating circuit configured for generating a firstactuation signal for a first switching element, used for applying aninput voltage to an inductive storage element, and a second actuationsignal for a second switching element, used as a first rectifier elementin a rectifier arrangement, in a step-up converter; and wherein theactuating circuit is configured so that during an actuation cycle itgenerates a turn-on level for the first actuation signal for a firstturned-on period and generates a turn-off level for the second actuationsignal during this first turned-on period, generates a turn-on level forthe second actuation signal at the end of the first turned-on period orafter the end of the first turned-on period for a second turned-onperiod, and monitors a current flowing through the second switchingelement or the inductive storage element during the second turned-onperiod, and terminates the second turned-on period after a change in thecurrent direction of this current from a first current direction to anopposite second current direction has been detected.